Add transport sync button and allow dropping stays onto band-covered days

- Auto-created transports carry auto_ref {from,to} stay ids; new
  POST /api/trips/:id/transports/regenerate reconciles them against the
  current stay order (re-date/re-title kept bridges preserving mode and
  price, delete orphans, create missing) without touching manual
  transports or flight-covered gaps
- Sync transports button in the calendar header with result toast
- The week band strip is now a drop target resolving the day from the
  pointer position, so stays can be dropped onto spots covered by other
  stays; overlapping stays stack in band lanes

public/js/views/dayEditor.js

@@ -137,6 +137,7 @@ export function openDayEditor(tctx, date) {
           time ? ` · ${time}` : '',
           flight ? ` · ✈️ ${flightChain(entry.segments)}` : '',
           !flight && entry.location_name ? ` · 📍 ${entry.location_name}` : '',
+          entry.auto_ref ? ' · ↻ auto' : '',
         ),
         isMultiDay(entry) ? el('div', { class: 'entry-span muted' }, spanText(entry)) : null,
         flight ? renderSegmentLines(entry.segments) : null,